To estimate a maximum purchase price for the coin in question, we can analyze several factors:

1. Current Condition: The coin has a condition of 70 (MS-). This indicates that it is in relatively good shape, but not among the highest quality.

2. Auction History:
- The average hammer price for coins of similar condition (70) in the past has been approximately 35-50 euros, based on previous auction hammer prices.
- In past auctions, coins with a condition of 70 have reached hammer prices of up to 110 euros (as seen in the auction on 2020-12-16).

3. Starting Price and Average Starting Price:
- The current starting price is 18 euros, which is quite low compared to the average starting price of 52.47 euros. This suggests that there is potential for the hammer price to be significantly higher.

4. Recent Trends:
- In the most recent auctions, the hammer price has varied, but there are examples where similar coins have reached hammer prices of 50-70 euros.

5. Data-Based Estimation:
- Given that the coin has a condition of 70 and considering the history of hammer prices, an estimated maximum purchase price could be between 50 and 80 euros. This is based on the fact that the hammer price can be higher than the average hammer price due to demand in the auction.

Conclusion: An estimated maximum purchase price for this coin could be 70 euros. This reflects a balance between the condition, price history, and market trend.